System and method for closed loop VSWR correction and tuning in RF power amplifiers

ABSTRACT

A system and method is provided to compensate an amplifier circuit for changes in a load impedance in order to maintain a substantially optimum performance for the amplifier. More specifically, if the load impedance increases, then the amplifier is reconfigured to produce an output impedance that is likewise increased. One way of reconfiguring the amplifier for a load impedance increase is to increase the supply voltage to the device. The increase in the supply voltage to the device increases the rail to rail operation of the device. This would allow more dynamic range for the system performance. Assuming the current is substantially constant, the impedance seen the output of the amplifier will increase and be multiplied up to the impedance desired by the load resulting in a more optimum power transfer. Other parameters, such as the input drive and bias voltage to the amplifier can be changed in order to improve the performance of the amplifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional patent application of Ser. No. 09/552,284,filed on Apr. 19, 2000, entitled “SYSTEM AND METHOD FOR CLOSED LOOP VSWRCORRECTION AND TUNING IN RF POWER AMPLIFIERS,” which is incorporatedherein by reference.

FIELD OF THE INVENTION

[0002] This invention generally relates to radio frequency(RF)/microwave amplifiers, and in particular, a system and method forautomatically improving the impedance match between a load and theoutput impedance of an output power stage.

BACKGROUND OF THE INVENTION

[0003] When an RF power amplifier is designed, the load variations aretaken into account when determining the quiescent biasing condition, or“Q” point, for the device. Depending on how poor the match can be with agiven load, the resultant “Q” point ends up being compromised. When thecondition exists that the magnitude of the load is less than the optimalor characteristic impedance expected by the amplifier, power transfer isno longer optimum.

[0004] To achieve the same output power at the load, in the mismatchedcondition, much more power is dissipated across the output stage. Thisincrease in the power dissipated causes excess heat to be generated inthe amplifier which results in an elevated device junction temperature.The elevated junction temperature has an exponential relationship withreliability. In addition to reliability degradation, more power must besupplied to the amplifier than would be required for a properly matchedcondition. For portable applications which rely heavily on batterycapacity, this can drastically increase the time required betweenrecharges.

[0005]FIG. 1 illustrates a block diagram of a prior art amplifier outputcircuit 100. The amplifier output circuit 100 consists of a field effecttransistor (FET) 102 having a grounded source (S), and a high RFimpedance bias network 104 which directs the power to the output stage.The inherent output impedance for power device 102 is usually much lessthan the impedance of the load 108. Therefore, for optimum performance,the impedance seen to the left of aa′ is transformed to substantiallymatch the impedance at the load 108. Typically, an impedance matchingnetwork 106 is employed to multiply up the output impedance of thetransistor 102.

[0006] The actual impedance of the transistor 102 is calculated bydividing the voltage drop across the transistor by the current flowingfrom drain (D) to source (S) at the quiescent bias point with no RFinput. This would equate basically to the supply voltage divided by thesupplied current to the stage in question. This is true for class Aoperation.

[0007] If the load impedance were to change for some reason, amismatched condition would exist. This would decrease the efficiency ofthe system since the criteria for optimum power transfer has beenviolated. The following discusses the effects of the mismatchedconditions when the load impedance increases and decreases.

[0008] If the load impedance increases, then the transistor's outputimpedance seen at aa′ will be too low for a matched condition. The accurrent flowing through the transistor 102, about the “Q” point currentwill decrease. The ac voltage, seen at the output, will increase acrossthe load constant. This can only occur until the device starts toapproach rail to rail operation. At this point, distortion begins toevidence itself and even though the power may start to approach thedesired output power level, the spectral density of this power maybecome very undesirable since the energy is no longer confined to thedesired signal but to intermodulation products as well.

[0009] If the load impedance decreases, then the transistor's outputimpedance seen at aa′ will be too high for a matched condition. The accurrent flowing through the transistor 102, about the “Q” point current,will increase. The ac voltage, seen at the output, wants to decrease.This can continue only until the device starts to saturate in itsability to provide increased current. At this point, distortion beginsto evidence itself and, even though the power may start to approach thedesired output power, the spectral density of this power may become veryundesirable since the energy is no longer confined to the desired signalbut to intermodulation products as well. In addition, there will be anincreased voltage drop across as well as current through the outputtransistor 102. This will cause an increase in junction temperaturewhich will lead to increased stress on the transistor 102 as well asperformance degradation.

[0010] Accordingly, there is a need to mitigate some of the problemsstated above.

SUMMARY OF THE INVENTION

[0011] One aspect of the invention is a method to compensate anamplifier circuit for changes in a load impedance in order improve theperformance of the amplifier. More specifically, if the load impedanceincreases, then the amplifier is reconfigured to produce an outputimpedance seen at aa′ that likewise increased. One way of reconfiguringthe amplifier for a load impedance increase is to increase the drain(FET) or collector (bipolar) voltage to the device. The increase in thedrain (FET) or collector (bipolar) voltage to the device increases therail to rail operation capability of the device. This would allow moredynamic range for the system performance. Assuming the drain (FET) orcollector (bipolar) current is substantially constant, the impedanceseen at aa′ will increase and be multiplied up, by the impedancematching network, to the impedance desired by the load resulting in moreoptimum power transfer.

[0012] Similarly, if the load impedance decreases, then the amplifier isreconfigured to produce an output impedance seen at aa′ that likewisedecreases. One way of reconfiguring the amplifier for a load impedancedecrease is to decrease the drain (FET) or collector (bipolar) voltageto the device. Assuming the drain (FET) or collector (bipolar) currentis substantially constant, the impedance seen at aa′ will decrease andbe multiplied up, by the impedance matching network, to an impedancecloser to that desired by the load for a more optimum power transfer.

[0013] In addition to changing the supply voltage to the amplifier fortuning its output circuit with the load, the drive input and the gatevoltage (FET) or base current (bipolar) to the amplifier can also bechanged to improve the impedance match between the output of theamplifier and the load. Specifically, if the load impedance increases,then the input drive to the amplifier is increased, and if the loadimpedance decreases, the input drive to the amplifier is decreased.Also, if the load impedance increases, the gate voltage (FET) or basecurrent (bipolar) to the amplifier is changed to decrease the conductioncurrent through the device, and if the load impedance decreases, thegate voltage (FET) or base current (bipolar) to the amplifier is changedto increase the conducting current through the device.

[0014] Other aspects of the invention includes an amplifier comprisingan output amplification stage, a directional coupler for generatingsignals indicative of forward and reverse powers between the outputamplification stage and a load, and a controller for determining animpedance match between the output amplification stage and the load fromthe signals indicative of forward and reverse powers. The controller iscapable of changing an input drive to the output amplification stage inresponse to a change in an impedance of the load to improve theimpedance match between the output amplification stage and the load. Thecontroller can also change the drain and/or gate voltages (FET), orcollector voltage and/or base current (bipolar) to the amplifier alongwith the input drive, individually or in any combination, to improve theimpedance match between the output amplification stage and the load.

[0015] An additional aspect of the invention includes a method of tuningan amplifier, comprising the steps of changing an input drive to theamplifier along with the drain and/or gate voltages (FET), or collectorvoltage and/or base current (bipolar) to the amplifier, individually orin any combination, to improve the performance of the amplifier. Thetuning of the amplifier by changing the variables listed above may be inresponse to a change in the load impedance. In such a case, it may bedesirable to determine the impedance match between the output of theamplifier and the load.

[0016] Still, yet another aspect of the invention includes an amplifiercomprising a plurality of cascaded amplification stages including anoutput amplification stage, a directional coupler for generating signalsindicative of forward and reverse powers between the outputamplification stage and a load, and a controller for determining animpedance match between the output amplification stage and the load fromsignals indicative of forward and reverse powers, and for changing theoperating conditions of respective amplification stages to improve theimpedance match between the output amplification stage and the load. Theoperating conditions of the respective amplification stages changedinclude the drain (or collector) voltage, input drive and/or the gatevoltage (or base current). A directional coupler may be included betweeneach of the stages of an amplifier, or between some of the stages.

[0017] Other aspects of the invention will become apparent from thedetailed discussion of the invention as provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 illustrates a block diagram of a prior art amplifier outputcircuit;

[0019]FIG. 2 illustrates a block diagram of an exemplary amplifier withan exemplary circuit for controlling the impedance match between theoutput of the amplifier and a load, in accordance with the invention;

[0020]FIG. 3 illustrates a block diagram of an exemplary multiple stageamplifier in accordance with the invention;

[0021]FIG. 4 illustrates a block diagram of yet another exemplarymultiple stage amplifier in accordance with the invention; and

[0022]FIG. 5 illustrates a block diagram of an exemplary amplifier thatis useful for various modes of operations, including a dynamic control,factory-set, user-selectable, and pre-programmed modes of operations.

DETAILED DESCRIPTION OF THE INVENTION

[0023]FIG. 2 illustrates a block diagram of an exemplary amplifier 200with an exemplary circuit for controlling the impedance match betweenthe output of the amplifier and the load in accordance with theinvention. The amplifier 200 comprises an output amplifier stage 202, adirectional coupler 204 at the output of the amplifier stage 202, acontroller 206, a controlled bias source 208, and a voltage variableamplifier (VVA) 210. The directional coupler 204 provides the controller206 the forward and reverse power at the output of the amplifier stage202. The controller 206 can determine the VSWR or quality of theimpedance match between the output of the amplifier stage 202 and theload (not shown). The controller 206 monitors the impedance matchbetween the output of the amplifier stage 202 and the load, andreconfigures the amplifier stage 202 to improve its power performance.The load can include passive elements, such as an antenna, an input to asubsequent cascaded amplification stage or an input to anotherelectronic device.

[0024] The controller 206 is coupled to the controlled bias source 208by way of line “a”. Through line “a”, the controller 206 controls thecontrolled bias source 208 so that a desired bias configuration isapplied to the amplifier stage 202. In the preferred embodiment, thebias configuration to the amplifier stage 202 may be adjusted byrelatively small step increments or decrements. Adjusting the biasconfiguration in steps allows the controller 202 to re-measure theoutput impedance match between steps, until a more optimum operatingcondition for the amplifier stage 202 is located. The bias configurationmay include drain and/or gate voltage to a field effect transistor(enhanced or depletion mode), or a collector voltage and/or base currentto a bipolar transistor. The controller 206 may also use line “a” tomonitor the current (e.g. drain or collector current) drawn by theamplifier stage 202. If too much current is drawn in a class ABamplifier stage 202, compensation may become necessary by lowering thecurrent to the amplifier stage 202.

[0025] The controlling of the bias of the amplifier need not be limitedto bipolar and field effect transistor technology. But, can beapplicable to other devices that are developed in the future. Inessence, what is being done here is the impedance of the active deviceis being changed to compensate for changes in the amplifier operatingenvironment, including variations in the load impedance, temperature,and input drive. This operation need not be limited to bipolar or fieldeffect transistor technology, and can encompass other technologiesincluding future developed technologies. Also other devices within thefamily of bipolar and field effect transistor technology arecontemplated, including heterojunction bipolar transistors (HBTs)(bipolar technology) and pseudomorphic high electron mobility transistor(PHEMTs) (field effect transistor technology).

[0026] The controller 206 is coupled to the voltage variable amplifier(VVA) 210 by way of line “b”. Through line “b”, the controller 206controls the gain or attenuation of the voltage variable amplifier (VVA)210. The voltage variable amplifier (VVA) 210 is used to change thedrive level to the amplifier stage 202. The voltage variable amplifier(VVA) 210 receives an RF input signal at a particular level. By havingthe controller 206 vary the gain or attenuation of the voltage variableamplifier (VVA) 210, the power level of the RF signal at the input ofthe amplifier stage 202 can be varied. This can be used to change the“Q” point for the amplifier stage 202 to improve the impedance matchbetween the output of the amplifier stage 202 and the load. This isparticularly true for class AB type amplifiers. An attenuator, such as apin diode attenuator, can also be used in place of the voltage variableamplifier (VVA) 210. Additionally, the VVA can also be used to controlthe output power and overall gain of the amplifier.

[0027] The controller 206 is coupled to the amplifier stage 202 by wayof line “c”. Through line “c”, the controller 206 receives a signalindicative of the temperature of the active device in the amplifierstage 202. The controller 206 uses the device temperature information toreconfigure the amplifier stage 202 to either operate at a more benignoperating condition or to shutdown when the device temperature exceeds apredetermined threshold. The benign operating condition or shutdowncontinues until the device temperature drops to an acceptable level.

[0028] In operation, if the load impedance increases, the controller 206senses the mismatched condition by monitoring the forward and reversepowers received from the directional coupler 204. The controller 206then instructs the controlled bias source 208 to increase the drain orcollector voltage to the amplifier stage 202 in incremental steps.Between adjacent steps, the controller 206 re-determines the impedancematching between the output of the amplifier stage 202 and the load. Thecontroller 206 adjusts the controlled bias source 208 until the drain orcollector voltage is reached that results in the desired impedance matchbetween the output of the amplifier stage 202 and the load. The voltagechanged can include the drain voltage if a type field effect transistoris used in the amplifier stage 202 or the collector voltage if a typebipolar transistor is used in the amplifier stage 202.

[0029] If the load impedance decreases, the controller 206 senses themismatched condition by monitoring the forward and reverse powersreceived from the directional coupler 204. The controller 206 theninstructs the controlled bias source 208 to decrease the drain orcollector voltage to the amplifier stage 202 in decremental steps.Between adjacent steps, the controller 206 re-determines the impedancematching between the output of the amplifier stage 202 and the load. Thecontroller 206 adjusts the controlled bias source 208 until the drain orcollector voltage is reached that results in the desired impedance matchbetween the output of the amplifier stage 202 and the load. Again, thebias voltage can include the drain voltage if a field effect transistoris used in the amplifier stage 202 or the collector voltage if a bipolartransistor is used in the amplifier stage 202.

[0030] Alternatively, if the load impedance increases, the controller206 senses the mismatched condition by monitoring the forward andreverse powers received from the directional coupler 204. The controller206 then increases the gain (or decreases the attenuation) of thevoltage variable amplifier (VVA) 210 in incremental steps to raise thedrive to the amplifier stage 202. Between adjacent steps, the controller206 re-determines the impedance matching between the output of theamplifier stage 202 and the load. The controller 206 continues toincrease the gain (or decreases the attenuation) of the voltage variableamplifier (VVA) 210 until desired impedance match between the output ofthe amplifier stage 202 and the load is achieved.

[0031] If the load impedance decreases, the controller 206 senses themismatched condition by monitoring the forward and reverse powersreceived from the directional coupler 204. The controller 206 thendecreases the gain (or increases the attenuation) of the voltagevariable amplifier (VVA) 210 in decremental steps to lower the drive tothe amplifier stage 202. Between adjacent steps, the controller 206re-determines the impedance matching between the output of the amplifierstage 202 and the load. The controller 206 continues to decrease thegain (or increase the attenuation) of the voltage variable amplifier(VVA) 210 until the desired impedance match between the output of theamplifier stage 202 and the load is achieved.

[0032] In an alternative embodiment, the amplifier stage 202 isconfigured so that the impedance match between the output of theamplifier and the load is substantially optimum when the input drive tothe amplifier stage 202 is relatively low. Then, when the load impedancechanges (i.e. either increasing or decreasing), the controller 206causes the voltage variable amplifier (VVA) 210 to increase in gain (ordecrease in attenuation) so that the input drive to the amplifier stage202 is increased. This action improves the impedance match between theoutput of the amplifier stage 202 and the load when the load increases.

[0033] Also alternatively, if the load impedance increases, thecontroller 206 senses the mismatched condition by monitoring the forwardand reverse powers received from the directional coupler 204. Thecontroller 206 then instructs the controlled bias source 208 to changethe gate voltage of a field effect transistor for decreased channelconduction or decrease the base current of a bipolar transistor of theamplifier stage 202 in steps. Between adjacent steps, the controller 206re-determines the impedance matching between the output of the amplifierstage 202 and the load. The controller 206 continues to instruct thecontrolled bias source 208 accordingly until the desired impedance matchbetween the output of the amplifier stage 202 and the load is achieved.

[0034] If the load impedance decreases, the controller 206 senses themismatched condition by monitoring the forward and reverse powersreceived from the directional coupler 204. The controller 206 theninstructs the controlled bias source 208 to change the gate voltage of afield effect transistor for increased channel conduction or increase thebase current of a bipolar transistor of the amplifier state 202 insteps. Between adjacent steps, the controller 206 re-determines theimpedance matching between the output of the amplifier stage 202 and theload. The controller 206 continues to instruct the controlled biassource 208 accordingly until the desired impedance match between theoutput of the amplifier stage 202 and the load is achieved.

[0035] The above three methods (i.e. changing the drain voltage, thedrive level, and the gate voltage for field effect transistoramplifiers, or changing the collector voltage, the drive level, and thebase current for bipolar amplifiers) of compensating for changes in theload impedance can be performed individually or in any combination.

[0036]FIG. 3 illustrates a block diagram of an exemplary multiple stageamplifier 300 in accordance with the invention. The amplifier 300comprises two or more amplification stages. In the example shown, theamplifier 300 includes “n” amplification stages. For each amplificationstage, the amplifier 300 includes a voltage variable amplifier (VVA) anda controlled bias source (CBS) coupled to the system power. The voltagevariable amplifiers (VVA) can vary the drive to respective amplificationstages, and the controlled bias sources (CBS) can vary the drain and/orgate voltages for respective field effect transistor amplificationstages, or the collector voltage and/or base current for respectivebipolar transistor amplification stages. The amplifier 300 furtherincludes a directional coupler 302 at the output of the last stage (i.e.stage “n”). The directional coupler provides the controller 304 theforward and reverse power at the output of the amplifier stage “n”. Thecontroller 304 can determine the VSWR or quality of the impedance matchbetween the output of the output stage (i.e. stage “n”) and the load(not shown).

[0037] The controller 304 monitors the impedance match between theoutput of the output stage (i.e. stage “n”) and the load, and tunes eachof the amplification stages (i.e. stages 1 through “n”) to achieve thedesired impedance matching between output amplification stage (i.e.stage “n”) and the load. The controller 304 tunes each of theamplification stages by changing the respective drain voltages (fieldeffect transistors) or collector voltages (bipolar transistors) with theuse of the respective controlled bias sources (CBS′), and/or by changingrespective drive inputs with the use of respective voltage variableamplifiers (VVAs), and/or by changing respective gate voltages (fieldeffect transistors) or base currents (bipolar transistors), whilemonitoring the impedance match between the output stage (i.e. stage “n”)and the load. Thus, the controller 304 can tune all or some of thestages of the amplifier 300 to achieve the desired performance for theamplifier 300. The amplification stages can be all be implemented withfield effect transistors or bipolars, or a mixture of bipolars and fieldeffect transistors.

[0038]FIG. 4 illustrates a block diagram of yet another exemplarymultiple stage amplifier 400 in accordance with the invention. Themultiple stage amplifier 400 is similar to multiple stage amplifier 300previously discussed, except that a directional coupler (i.e.directional couplers 402-1, 402-2 and 403-3) is at the output of each ofthe amplification stages in order for a controller 404 to monitor theimpedance match between amplification stages. The controller 404 tuneseach of the amplification stages by changing the respective drainvoltages (field effect transistors) or collector voltages (bipolartransistors) with the use of the respective controlled bias sources(CBS′), and/or by changing respective drive inputs with the use ofrespective voltage variable amplifiers (VVAs), and/or by changingrespective gate voltages (field effect transistors) or base currents(bipolar transistors), while monitoring the impedance matches betweenstages and between the output stage and the load. Thus, the controller404 can tune all or some of the stages of the amplifier 400 to achievethe desired performance for the amplifier 400. The amplification stagescan be all be implemented with field effect transistors or bipolars, ora mixture of bipolars and field effect transistors. Although in theexemplary multiple stage amplifier 400 includes is a directional couplerbetween each of the stages, it shall be understood that there need notbe a directional coupler between every cascaded stages.

[0039] In the exemplary embodiments, the tuning of the amplificationstages can be performed in a manner that the operating class of theactive device can be changed. For example, if greater output linearityof an amplification stage is desired, its corresponding drain orcollector voltage, input drive, and/or gate voltage or bias current maybe changed to reconfigure the amplification stage from operating in aclass “A” condition to a class “AB” condition, a class “B”, or a class“C”. More generally, the operating condition of the amplifier can bechanged so that the amplifier is reconfigured from operating in a classto operating in another class. In this manner, the amplification stageor amplifier can be re-configured to operate in a more desired manner.

[0040] The above exemplary embodiments of the invention have beengenerally directed at dynamic controlling of the operating conditions ofan amplifier to achieve a desired result in the midst of changes invarious environment parameters, including load impedance, temperature,and input drive variations. There are other modes of operations, apartfrom the dynamic control operation, for the above amplifiers. These arefactory-set operating condition amplifiers, user-selectable operatingcondition amplifiers, and pre-programmed operating condition amplifiers.

[0041] In factory-set operating condition amplifiers, a computer orother processor-based system is used in a similar manner as controller206 to determine a desired operating condition for the amplifier given aknown set of conditions. More specifically, in the factory, a computeror other processor-based system is used to set a desired operatingcondition by varying the bias and input drive to the amplifier inaccordance with the invention as described above. Once the desiredoperating condition for the amplifier has been determined, datarepresenting the desired operating condition can be rewritten into anon-volatile memory (e.g. a flash EEPROM) to be accessed by a controllerupon powering the amplifier. The controller then uses the data to setthe amplifier at the desired operating condition.

[0042] In a user selectable operating condition amplifier, the amplifierwill be used in several distinct applications where the desiredoperating condition for the amplifier differs. In a factory setting, acomputer or other processor based system is used to set the desiredoperating condition for each of the applications for the amplifier. Datarepresenting the various operating conditions is written into anon-volatile memory to be accessed by a controller for varying theoperating condition of the amplifier. A user can select a desiredapplication for the amplifier by activating a switch or the like, andthe controller responds accordingly by accessing the data relating tothe corresponding operating condition from the non-volatile memory. Thecontroller then uses the data to set the amplifier at the correspondingoperating condition.

[0043] In a pre-programmed operating condition amplifier, the desiredoperating condition for an amplifier may depend on many environmentvariables, including temperature, load impedance, and input drive, toname a few. In a factory setting, a computer or other processor basedsystem is used to determine the desired operating conditions forrespective sets of environment parameters. These various operatingconditions and corresponding environment variable sets can be writteninto a non-volatile memory in a look-up table fashion to be accessed bya controller to set the operating condition of the amplifier. Duringoperation of the amplifier, a controller monitors the environmentparameters of the amplifier, and then searches the look-up table in thememory to access the corresponding operating condition for theamplifier. The controller then sets the amplifier to the selectedoperating condition.

[0044]FIG. 5 illustrates an exemplary amplifier 500 that is useful forthe various mode of operations, including the dynamic control,factory-set, user-selectable, and the pre-programmed amplifiers.Amplifier 500 is similar to amplifier 200 (FIG. 2) in that it comprisesan amplification stage 502, an output directional coupler 504, acontroller 506, a controlled bias source 508, and a voltage variableamplifier (VVA) 510. The amplifier 500 further includes a non-volatilememory 512 (e.g. a flash EEPROM) for storing data relating to one ormore operating condition settings for the amplifier 500. The data may beaccessed by the controller 506 for setting the amplification stage 502to the selected operating condition. The amplifier 500 may also includea directional coupler 514 at its input to allow the controller 506 todetermine the input matching.

[0045] Amplifier 500 lends itself to mass production since a pluralityof this type of amplifiers can be configured into a bank, and theoperating conditions for each of the amplifiers can be automatically setand written into the non-volatile memory 512 by a dedicated computer orprocessor-based system. Amplifier 500 also lends itself to servicing inthe field with the use of a portable computer or other processor-basedsystem that can update the operating condition(s) data stored in thenon-volatile memory 512.

[0046] In the foregoing specification, the invention has been describedwith reference to specific embodiments thereof. It will, however, beevident that various modifications and changes may be made theretodeparting from the broader spirit and scope of the invention. Thespecification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

It is claimed:
 1. A method of tuning an amplifier having a plurality of amplification stages, comprising: automatically determining an impedance match between an output of one of said amplification stages and a load; and changing an input drive to said one of said amplification stages to tune said amplifier to achieve a desired performance for said amplifier.
 2. The method of claim 1, wherein said load includes an input impedance of a next cascaded amplification stages.
 3. The method of claim 1, wherein said changing of said input drive comprises modulating said input drive to said amplifier to maintain a substantially constant output power level for said amplifier.
 4. The method of claim 1, further including changing a channel conductance or base current to said amplifier to achieve said desired performance for said amplifier.
 5. The method of claim 4, wherein said changing said channel conductance or base current comprises decreasing said channel conductance or base current to said amplifier if said load impedance increases.
 6. The method of claim 4, wherein said changing said channel conductance or base current comprises increasing said channel conductance or base current to said amplifier if said load impedance decreases.
 7. The method of claim 1, further including changing a drain or collector voltage to said amplifier to achieve said desired performance for said amplifier.
 8. The method of claim 7, wherein said changing said drain or collector voltage comprises a step of increasing said drain or collector voltage to said amplifier if said load impedance increases.
 9. The method of claim 7, wherein said changing said drain or collector voltage comprises a step of decreasing said drain or collector voltage to said amplifier if said load impedance decreases.
 10. The method of claim 7, wherein: said changing said input drive to said amplifier comprises of: modulating said input drive to said amplifier to maintain a substantially constant output power level for said amplifier; and further including: changing a channel conductance or base current to said amplifier to improve said impedance match between said output of said amplifier and said changed load impedance, wherein said changing said channel conductance or base current comprises decreasing said channel conductance or base current to said amplifier if said load impedance increases, and increasing said channel conductance or base current to said amplifier if said load impedance decreases; and changing a drain or collector voltage to said amplifier to improve said impedance match between said output of said amplifier and said changed load impedance, wherein said changing said drain or collector voltage comprises increasing said drain or collector voltage to said amplifier if said load impedance increases, and decreasing said drain or collector voltage to said amplifier if said load impedance decreases.
 11. The method of 1, further including: determining a temperature of an active device of said amplifier; and reducing a supply power to said amplifier if said temperature exceeds a predetermined level.
 12. The method of claim 11, wherein said reducing said supply power comprises reducing said supply power until substantially no current is drawn by said active device.
 13. An amplifier, comprising: a plurality of cascaded amplification stages including an output amplification stage; a directional coupler for generating signals indicative of forward and reverse powers between said output amplification stage and a load; and a controller for determining an impedance match between said output amplification stage and said load from said signals indicative of forward and reverse powers, and for changing operating conditions of respective amplification stages to improve said impedance match between said output amplification stage and said load.
 14. The amplifier of claim 13, wherein said controller changes operating conditions of respective amplifier by changing drain or collector voltages, input drives and/or drain or base currents of respective amplification stages to improve said impedance match between said output amplification stage and said load.
 15. The amplifier of claim 13, further including a second directional coupler to generate a second set of signals indicative of forward and reverse powers between cascaded amplification stages, and wherein said controller determines said impedance match between said cascaded amplification stages from said second set of signals. 